This invention relates generally to hard disk drive control and, more particularly, to seek trajectory generation using adaptive filtering.
Computer hard disk drives (HDDs) include one or more disks of magnetic storage medium and a disk drive head assembly to read and write data on the magnetic storage medium. Read/write heads typically include a write element comprised of a thin film inductive head and a read element comprised of a Magnetoresistive (MR) sensor. The heads for the disk surfaces of the disk drive are affixed to an actuator or arm that glides across the disk surface to position the head at different track locations. Current is passed to a voice coil motor (VCM) to position the actuator with respect to the disk surface. The amount of torque applied to the actuator is governed by the amount of current in the VCM. The VCM comprises the coil that receives the current and two magnets. During operations, the disk drive components, such as the VCM, can produce vibrations and oscillations induced as a result of the resonance of the components. Such vibrations may result in undesirable head variations and tracking errors.
Two factors that affect the access time that lapses before the head can access the disk include move time and settle time. The move time is the time required for the head to move between tracks. The settle time is the amount of time required, after the actuator has moved the head assembly during a seek, for the heads to stabilize sufficiently for the data to begin to be read or written. The characteristics of the disk drive system and environment can produce oscillations that may increase the settle time and thereby degrade disk performance. Further, error may result if the head overshoots or undershoots the desired track.
In current disk drive systems, to correct for tracking errors resulting from noise and vibrations from components such as the VCM, the disk drive controller will read servo information indicating the actual current position and compare that value read to the desired position. This difference is known as the Position Error Signal (PES). The drive controller will then calculate a current to apply to the VCM to correct any variation in the measured position versus the desired position. Thus, the position feedback controller calculates the desired current to be applied to the VCM to generate torque to move the actuator arm. The current is supplied by an amplifier between the feedback controller output and the VCM input.
In some cases, the seek trajectory is optimized for seek speed to move the actuator arm quickly, but it is also desirable for the head to settle quickly over the correct data track. A typical approach is the bang-bang approach in which, for instance, a positive current moves the head to the new position and a negative current brakes the head at the new position. Such a current will appear as two step function pulses of opposite polarity (i.e., a single square wave). The detailed physics involved, however, makes the use of the bang-bang approach a non-ideal solution. A number of small modifications have been used for bang-bang VCM current vector to improve its performance by moving the head to the new track quickly while minimizing the settle time at the desired track. One example involves the Fourier seek method as described in U.S. Pat. No. 6,549,364.
During an HDD seek operation, the actuator moves the head to the target position as quickly as possible. To reduce the seek time, the amplitude of the current is increased using a generally square or rectangular waveform. This will generate RTV (Random Transient Vibrations) due to mechanical motion and acoustic noise from propagation of the vibration. It is desirable to design the seek control trajectory with a smoother waveform to produce smaller RTV and/or smaller seek acoustic noise, but the existing design method involves trial and error and it may not be optimum for specific actuator/drive dynamics.